Effect of nanoparticles on the mechanical properties of kenaf fiber-reinforced bio-based epoxy resin

Abstract

Ecological composites materials have become a topic of interest in materials science and engineering in recent years because of the growing need for environment-friendly composites that maintain properties like light weight, but gain biodegradability and renewability. Bio-composites of kenaf fiber-reinforced bio-based epoxy resin filled with different kind of nanoparticles were prepared in this work using a hybrid manufacturing process combining vacuum-assisted resin infusion and an autoclave. Nanoparticles of carbon, metal oxides and layered silicates were employed in this work to analyze their effect on the mechanical properties of bio-kenaf composites. Nanoparticles were synthesized and heat-treated according to their nature, and Fourier transform infrared spectra confirmed the presence of functional groups. Laminar structures, such as graphene or layered silicates, had more influence on the bio-kenaf composite toughness than the particle-like morphology of metal oxides. However, the nanoparticles influenced the strength because of their effective stress transfer mechanism. Despite voids of different sizes, which were detected using scanning tomography, they did not influence the mechanical properties of the bio-kenaf composites, showing that the filler effect of the nanoparticles is the dominant mechanism.

Keywords

mechanical properties nanoparticles bio-composites natural fibers bio-epoxy

The development of environmentally friendly composite materials has become a topic of interest in materials science and engineering in recent years.^1–3 Natural fibers present social and economic advantages and reduced costs and energy consumption in comparison with synthetic fibers. Fabrics of kenaf (Hibiscus cannabinus)^4–8 are a sustainable alternative for the development of ecological composites oriented to semi-structural components because of their cellulose, hemicellulose and lignin content.^9,10 Sathyaseelan et al.¹¹ observed that short fibers of kenaf can provide stiffness and good thermal behavior to epoxy resins. According to the literature,^9,10,12 kenaf fibers have a tensile strength from 350 to 600 MPa, Young’s modulus from 4 to 12.5 GPa and an elongation break from 2.5% to 3.5%, with a density of 1.2–1.5 g/cm³. These properties have attracted attention to develop hybrid components and parts oriented to the construction, automotive, marine and sports sectors.

As a matrix, bio-based epoxy resins are an environmentally friendly option because they are produced from epoxidized vegetable oil (EVO), which contains oxirane groups that are highly reactive.^13,14 EVO resins are synthetic materials that consist partially of renewable raw materials up to a maximum of 50%. However, the higher the bio-based content, the lower the mechanical properties, causing higher heat to be produced by the crosslinking reaction.^15–17 An alternative to increase the mechanical properties of EVO resin is the addition of nanofillers.^18–20 Nanoparticles can also favor the thermal stability and provide multifunctional properties without modifying the density or altering the manufacturing process. However, there are extensive options available for nanofiller selection. The nature of nanoparticles is key in order to understand how much the nanoparticles influence the mechanical behavior of fiber-reinforced polymers (FRPs). Despite the extensive literature about the effect and influence of nanoparticles on several properties of composites,^{4,5,19,21–30} the comparison among different kinds of nanoparticles, as well as their effect on the mechanical properties of kenaf-reinforced EVO resin, has not been reported. Most of the particles show an excellent reinforcement effect at very low weight percentages of around 1–2 wt.% and several research articles have tested the effect of filler contents between 0 and 5 wt.%; the best results for the enhancement of mechanical properties are around 1 wt.%.

The nanoparticles commonly reported as composite reinforcements are carbon, metal oxides and ceramic nanostructures. There are several allotropes of carbon that are used as effective mechanical reinforcement of composites, such as carbon nanotubes, nanodiamonds and graphene. The last one presents variants as graphene oxide (GrO) that are compatible with different polymer resins.^26,31–36 Clay minerals are phyllosilicates with tetrahedral (T) and octahedral (O) layer structures that can be dispersed at the nano-scale level, promoting significant improvements in the polymer properties, which include enhanced mechanical strength, a gas barrier, thermal stability or flame retardancy.^20,37 Metakaolin (MK) is a phyllosilicate with a T:T structure that allows intercalation of the polymer chains and restrains deformation mechanisms.^38–40 Metal oxide particles improve many physical and thermal properties of thermosetting resins. Metal oxides such as Al₃O₂, FeO, SiO₂ and TiO₂ are widely used as nanofillers because they exhibit a high surface area to volume ratio that promotes a good surface interaction with polymer chains.^23,41–46 Some metallic oxides, such as zinc and copper, have a very simple and non-toxic process of synthesis.^24,47–51

It is possible to find extensive literature related to nanoparticles as fillers in composite materials. However, the effect of different kinds of nanofillers used as mechanical reinforcement of ecological composites has not been reported. In this work, organic GrO and inorganic particles, such as MK, zinc oxide (ZnO) and copper oxide (CuO), were used to evaluate their effect on the mechanical properties of kenaf fiber-reinforced EVO resin. This research aims to encourage the use of kenaf fiber as a potential material for non-structural components in the automotive and construction industries, and particularly for cabinets in aircraft cabin interiors.

Methodology

Materials

In this study, bobbins of three-ply yarn made up of kenaf fibers were obtained from Juteko Co., and were woven in a plain weave fabric configuration with a 200 GMS by Mexican artisans from the state of Queretaro in Mexico. The kenaf yarn, with a linear density of 2500 tex, has an elastic modulus of 10 GPa and tensile strength of 110 MPa, which were previously calculated following the methodology of the ASTM C1557 standard, and a density of 1.31 g/cm³, according to the ASTM D3822 procedure.

Surf Clear EVO with a bio-based carbon content of about 40% and SD EVO fast hardener from Sicomin Epoxy Systems® were used in a mixing ratio by volume of 2:1. According to the datasheet, EVO is a bio-epoxy resin with a tensile modulus and breaking strength of 3.4 GPa and 66 MPa, respectively.

Graphite powder (purity > 90.0%) from CIVEQ Reactives® and other reactive materials, such as potassium permanganate (KMnO₄ > 99.0%) from CIVEQ Reactives® and sulfuric acid (H₂SO₄ > 95.0%) and hydrochloric acid (HCl 36.0%) from MEYER Reactives®, were used to produce GrO.

MK Metastar 501 from Watson Phillips® is a calcined kaolin clay with a mean particle size of 1.2 µm and a specific gravity of 2.5 g/cm³.

Zinc acetate dehydrate (Zn(O₂CCH₃)₂·2H₂O > 99.4%) from J.T. Baker® and sodium hydroxide (NaOH > 97.0%) form Meter Reactives® were used for preparation of ZnO nanoparticles.

The synthesis of CuO (CuO I > 97.0%) from Sigma Aldrich® required hydrochloric acid (HCl 36.0%) from MEYER Reactives®, sodium hydroxide (NaOH > 97.0%) from J.T Baker® and potassium nitrate (KNO₃ > 99.0%) from Golden Bell Reactives®. All reagents used in this work were of analytical grade.

Synthesis of nanoparticles

GrO was prepared according to the Hummer method.⁵² In detail, 3 g of powder graphite was mixed with 75 ml of H₂SO₄ at a temperature below to 20℃ and stirred over 15 min to ensure a homogeneous dispersion. Next, 9 g of KMnO₄ was slowly added to the acidic solution and kept under constant stirring for 3 h at 15℃. To start the graphite oxidation, 150 ml of H₂O drop by drop was added, avoiding temperatures above 50℃. Once a color change from black to light brown was obtained, 100 ml of distilled water was added. Finally, 15 ml of H₂O₂ was added to remove the excess of KMnO₄ and it was washed with HCl and ethanol, then it was neutralized until pH 7. The reaction powder was exfoliated using ultrasonication for 180 min at an amplitude of 50%. For surface treatment, the powder was mixed in a solution of HNO₃ and H₂SO₄ in a 3:1 ratio and neutralized with ethanol.

MK was used after heat treatment at 800℃ for 2 h and placed into a desiccator for at least 12 h prior to adding to EVO resin.

ZnO was synthesized by the co-precipitation method⁵³ using 0.2 molars of zinc acetate dehydrate in 250 ml of ethanol, followed by stirred for 30 min at 70℃. Then, 0.5 molar of NaOH in 250 ml of ethanol was prepared and added drop by drop to the first solution. The mixed solution was stirred continuously, and the reaction temperature was controlled at 75℃. The resultant precipitates were collected by filtration and rinsed with de-ionized water to make sure that the residual impurities were removed. Subsequently, the washed precipitates were dried for 6 h at 75℃ in a conventional oven. The powder obtained was sonicated for 10 min and dried at 110℃ prior to mixing with EVO resin.

The CuO nanostructure was synthesized via the oxidative hydrolysis method.⁵⁴ Firstly, CuCl₂ was dissolved in 400 ml de-ionized water and placed in a reflux system for 15 min at 90℃; secondly, 6 ml of H₂O was prepared with 3.6 g of KNO₃ and 3.2 g of KOH and added to CuCl₂ drop by drop until reaction. The color of the solution turned from green to black as the reaction proceeded. The precipitate obtained was washed with de-ionized water and dried for 24 h at 120℃ in a conventional oven.

Manufacturing

In this work, the kenaf fabrics were washed carefully using distilled water to remove dust or contaminants and to maintain the fiber surface composition. Afterwards, the fabric was dried at 35℃ for 48 h and desiccated for 24 h to remove moisture. Subsequently, a handcraft layer of kenaf fabric was impregnated with EVO resin employing a hybrid manufacturing process that combines vacuum-assisted resin infusion (VARI) and an autoclave, as schematized in Figure 1. The VARI for impregnation and the autoclave for controlling pressure at 40℃ for 2 h were used during the curing of the composites.

Figure 1.

Schematic representation of the incorporation of nanoparticles to kenaf fiber-reinforced epoxidized vegetable oil using the vacuum-assisted resin infusion process for impregnation and an autoclave for the curing process.

Kenaf/EVO (KE) bio-composites were reinforced with 1 wt.% of nanoparticles identified as GrO, ZnO, MK and CuO. The nanoparticles were dispersed into the EVO resin, before the impregnation process, utilizing the sonication process using a Q700 Sonicator from Qsonica. Bio-laminates of 3.00 ± 0.5 mm thickness were obtained.

Characterizations

The structural analysis of KE bio-composites filled with different kinds of nanoparticles (from now on KE nanocomposites) was performed by Fourier transform infrared spectroscopy (FTIR) using a Perkin Elmer Frontier. The specimens were placed over the mid-infrared attenuated total reflectance (MIR ATR) optics assembly to the instrument. FTIR spectra were recorded by scanning the samples in the frequency range of 400–4000 cm⁻¹. The specimens were scanned for 32 scan times in the transmittance mode at a resolution of 4 cm⁻¹. Nanoparticles were analyzed by using the X-ray diffraction (XRD) technique to find their crystalline phases. The patterns were recorded in a Rigaku D/max-2100 diffractometer (Cu kα radiation, 1.5406 Å) in the range of 20–75° for an incidence angle of 0.5°. Dynamical mechanical analysis (DMA; DMA Discovery 850 from TA Instruments) was used to find the viscoelastic behavior of unfilled KE and KE nanocomposites. The storage modulus (E’) and damping factor (tan δ) curves were obtained by testing specimens with 50 mm × 12 mm × 3 mm dimensions following the ASTM-D7028 standard. The tests were performed in a single cantilever geometry to conduct measurements from room temperature to 120℃ with a heating ramp of 5℃/min. Unidirectional tensile tests were performed following the ASTM D3039 – 3039 M standard to calculate the tensile properties of unfilled KE and KE nanocomposites. Eight specimens with nominal dimensions of 250 mm × 25 mm × 2.5 mm and a gauge length of 150 mm were tested in a universal testing machine, Instron® 647, with 0.85% of calculated limiting error and equipped with a load cell of 30 kN to detect the applied load. Testing was performed under displacement control at a crosshead speed of 2 mm/min (0.07 in/min) and the stress versus strain curve was recorded until failure of the tensile tests. The uncertainty of measurement results for the tensile parameters were estimated using eight specimens taken from the same composite laminate. All specimens showed similar tensile curve shapes after testing, and none of them were eliminated from the study. The mechanical parameters reported in this work resulted from the average number and standard deviation. A video-extensometer, MTS Advantage Video Extensometer (AVX), with a 25 mm lens was employed to record elongation for each test by a non-contact technique. The video-extensometer recognizes patterns on surfaces to acquire measurement data for strain calculations, which were processed by MTS TestSuite™. X-ray computed tomography (CT) is a non-destructive technique for both visualizing interior features within solid objects and obtaining digital information on their three-dimensional (3D) geometries and physical properties. The GE Phoenix v|tome|x m is a CT scanner system with dual-tube technology that is used in this work to reveal manufacturing defects and quantify voids present in all KE nanocomposites with rectangular dimensions of 80 mm × 6 mm × 3 mm.

Results and discussion

Structural analysis

FTIR was used to perform a qualitative (identification) analysis to find the chemical interaction of the synthesized and heat-treated nanoparticles. We use the FTIR technique to find the surface characteristics of each particle and the possibility to enhance the interface interaction between those particles and the polymeric chains. This information may be an aid to the interpretation of the mechanical tests and other phenomena that could affect the performance of the composites. The FTIR spectra of GrO, MK, ZnO and CuO are shown in Figure 2. The FTIR spectrum corresponding to GrO nanoparticles is at the bottom of Figure 2. The broad band at 3300 cm⁻¹ is attributed to the stretching of the N–H aliphatic primary amine, while the bands around at 2900 and 2840 cm⁻¹ represent the stretching of the C–H bond. The band at 1740 cm⁻¹ is related to the C = C stretching vibration bond and the band at 1640 cm⁻¹ corresponds to amine –NH2. Besides, the band observed at 1550 cm⁻¹ is related to flexural vibration of a secondary amine. Weak bands containing carbon atoms detected at around 1500 and 1150 cm⁻¹ could be attributed to the C–OH stretching vibration and C–O stretching vibration.^55,56 Also, the highly intense signal located at the 1020 cm⁻¹ band is attributed to C–O tension vibration, and the weak band at 880 cm⁻¹ corresponds to C–H tension vibration. Through the spectra and the functional groups found, it is possible to confirm the presence of oxygen atoms on the graphene surface. The structure of this carbon allotrope is confirmed in Figure 3 by the XRD technique. Even when the signals are weak, functional groups were observed as the bonding between carbon and oxygen, carbon and nitrogen and some others, such as amine groups, which improves the interface interaction with polymers. Other works are well in agreement with our results.^18–20

Figure 2.

Fourier transform infrared spectra of different kind of nanoparticles used as reinforcement of kenaf/epoxidized vegetable oil bio-composites.

Figure 3.

X-ray diffraction patterns corresponding to different kinds of nanoparticles used as reinforcement of kenaf/epoxidized vegetable oil bio-composites: (a) graphene oxide (GrO); (b) metakaolin (MK); (c) zinc oxide (ZnO); (d) copper oxide (CuO).

The MK particle spectrum shows three characteristic bands at 1076, 797 and 467 cm⁻¹, which correspond to Si–O symmetric vibration, Al–O stretching vibration and Si–O bonding, respectively.^28,57 The absence of representative bands at around 3500–3700 cm⁻¹, typical for kaolin, suggests that the heat treatment was effective to obtain calcined kaolin.³⁸

For the case of ZnO nanoparticles, the band at 1735 cm⁻¹ is attributed to the stretching vibration of the C=O bond. The band at 1570 cm⁻¹ represents the N–H bond stretching vibration, and the peak located at 1430 cm⁻¹ corresponds to C–H stretching vibrations, which come from the acetate group. The stretching vibration of Zn–OH at 900 cm⁻¹ and the Zn-O peak at 450 cm⁻¹ suggests ZnO formation.^24,30,58 In this way, we have confirmed the formation of ZnO with -OH groups around its surface. These could be an anchor point for the polymeric chains.

The spectrum of CuO reveals weak signals around at 3478 cm⁻¹ ascribed to O-H stretching. The bands located at 1373 and 820 cm⁻¹ are assigned to the vibrations of the –NO3 and –HCO3 groups, respectively. Both signals are attributed to be residues from synthesis.³⁴ The signals located at 605 and 476 cm⁻¹ were ascribed to CuO vibration^55,56 and confirm the successful formation of CuO.^50,59

XRD is a valuable method to investigate the inter-layer changes and the crystalline properties of nanoparticles. Figure 3 shows the XRD spectra corresponding to the GrO, MK, ZnO and CuO nanoparticles. The XRD pattern of GrO is presented in Figure 3(a). The characteristic pattern of graphene shows a wide and medium intensity peak with a poorly defined maximum. The peak located at 2θ°∼ 21.3° indicates a lack in the interconnection of the carbon structure with oxygen atoms. Some authors refers to this peak as characteristic of GrO nanoplatelets with the presence of NH₃.³⁴ Furthermore, GrO nanoplatelets refer to hexagonal crystal structures from the P-6m2 space group, with sp2 hybridization.^29,60

Table 1 presents the parameters calculated from the XRD patterns for the GrO, MK, ZnO and CuO nanoparticles. Despite the diffraction peak position commonly being used to refer to the inter-layer spacing, the peak width is highly representative of graphitic size domains inside graphene platelets. Domains are laterally spaced by grain boundaries (graphitic breaking stacks) and longitudinally by sp2/sp3 domain boundaries.^29,60 Thus, the average crystallite width represents the out-of-plane crystallite size, which is associated with the perpendicular dimension of the graphite layer stacking order.

Table 1.

X-ray diffraction parameters

Nanoparticles	Peak position [2θ]	Miller indices	FWHM	Crystal size [nm]	Mean crystal size [nm]
GrO	21.35	(006)	1.27	0.92	0.92
	26.52	(001)	1.27	6.42
MK	42.70	(112)	1.68	5.04	5.73 ± 0.7
	31.66	(100)	0.26	31.53
	16.87	(002)	0.08	95.03
	34.32	(101)	0.26	31.55
ZnO	36.15	(102)	0.27	30.65	39.58 ± 1.5
	47.43	(110)	0.26	32.21
	56.49	(103)	0.26	33.56
	62.75	(200)	0.28	32.40
	67.83	(112)	0.28	33.33
	68.97	(201)	0.26	35.99
	29.47*	100*	0.21*	38.85*
	32.41	110	0.33	24.85
	35.43	002	0.49	17.00
	36.32*	111*	0.26*	32.14*
	38.62	200	0.57	14.61
	42.20*	112*	0.28*	29.57*
	48.65	–202	0.59	14.68
CuO	53.40	020	0.52	16.90	18.94 ± 9.12
	58.09	202	0.58	15.66
	61.31	-113	0.47	19.44
	65.99	022	0.89	10.59
	67.91	220	0.64	14.88
	73.40*	113*	0.45*	21.57*
	74.98*	311*	0.77*	12.99*
	78.90*	004*	28.65*	0.34*

FWHM: full width at half maximum.

XRD on crystalline powders offers a convenient method for determining the mean size of single crystal nanoparticles. The particle size (D) can be estimated from the full width at half maximum (FWHM) of the peaks in the XRD patterns using the Debye–Scherrer equation, as expressed in the following equation

D = \frac{K λ}{β cos θ} = \frac{0.89 λ}{B cos θ}

(1)

where β is the FWHM of the experimental diffraction signals expressed in radians, θ is the half diffraction angle of the peak, which in the case of graphene platelets or layered silicates corresponds to inter-layer spacing, λ is the wavelength (1.5418 Å) and K is a constant related to the crystallite shape. Some authors point out that the value of the K constant depends on the geometry particle, being 0.89 for a spherical crystal with a cubit unit cell or 0.943 when the particles have cubic geometry.⁶¹

According to Equation (1) the FWHM determined from GrO particles is 1.27 (Table 1) and using the Debye–Scherrer equation the crystallite size calculated is 0.92 nm.

The characteristic peaks of kaolinite commonly located at 2θ = 8°, 12°, 24° and 26° are not present in the XRD pattern of calcined kaolin presented in Figure 3(b); instead, a broad peak with a halo at 2θ between 25° and 30° is consistent with the presence of an amorphous quartz precursor, which is representative of MK.^38,62 From Debye–Scherrer calculations, the mean MK crystallite size is 5.73 nm.

The XRD pattern of nano ZnO particles is presented in Figure 3(c). The well-defined pattern allows one to detect peaks located at 2θ values of 31.8°, 34.4°, 36.2°, 47.5°, 56.6°, 62.9°, 66.4°, 67.9°, 69.1°, 72.6° and 76.9°. The reflection peaks can be indexed to the ZnO wurtzite-like structure with lattice constants for a hexagonal structure of a = b = 2.53 Å and c = 52.10 Å.⁶³ From Equation (1) the crystallite size was found to be 39.5 nm.

Figure 3(d) visualizes the XRD pattern of CuO. The peak positions at 2θ values of 32.41°, 35.43°, 38.62°, 48.65°, 53.40°, 58.09°, 61.31°, 65.99° and 67.91° allow one to assume that CuO adopts a monoclinic structure (C2/c space group) with the following lattice constants: a = 4.69 Å, b = 3.42 Å and c = 5.13 Å.^41,64 The average crystallite size of the synthesized CuO nanoparticles is close to 20 nm, which agrees with that calculated by other authors.⁴⁴ Furthermore, the XRD pattern of CuO nanoparticles matches the monoclinic phase of the tenorite structure^65,66 indexed to the peaks at 29.47°, 36.32°, 42.20°, 73.40° and 74.98°, and marked with an asterisk in Table 1.

Viscoelastic properties

DMA provides relevant information on the viscoelastic behavior of polymers, as well as on the thermal transitions. The storage modulus is a measure of stress stored in the sample as mechanical energy, while the damping factor is a typical measure of energy dissipation. DMA curves corresponding to the storage modulus and damping factor are presented in Figure 4. The typical storage modulus curve for thermosetting resins contains three regions: the glassy region, the glass transition and the rubbery plateau. In this work, the storage modulus in the glassy region varies with the addition of different kinds of nanoparticles, as shown in Figure 4(a). The storage modulus of KE-GrO and KE-MK remains practically constant previous to the onset point (glass transition temperature, T_g). In contrast, KE-ZnO and KE-CuO displayed diffusive regions, similar to the unfilled KE, where the storage modulus decreases gradually with temperature. For all specimens, the stiffness decreases by several factors in the glass transition region because of segmental motions until the rubbery plateau.

Figure 4.

Dynamical mechanical analysis results of unfilled kenaf/epoxidized vegetable oil (KE) and KE nanocomposites: (a) storage modulus; (b) tan δ.

The storage modulus of the unfilled KE was 2350 MPa in the glassy region, and the addition of fillers leads to an increase up to 3915, 3070 and 2730 MPa for KE-GrO, KE-MK and KE-ZnO, respectively. The lowest storage modulus value was found in KE-CuO (2280 MPa). The results obtained imply the effective reinforcement effect promoted by the presence of rigid nanoparticles that act as a secondary reinforcement within the EVO matrix. On the other hand, CuO nanoparticles can act as plasticizers, which contributes to load sharing but does not influence the stiffness of the KE bio-composites. The previous is in concordance with others authors, who reported that the shape and the low CuO nanoparticle concentrations influence the reinforcement mechanism of composites.^27,61

The stiffness in the rubbery plateau region presents some differences that are attributed to the kind of nanoparticles used as reinforcements and is proportional to the crosslinking density. The unfilled KE shows a storage modulus value of 208 MPa at approximately 100℃. At this temperature, the moduli of the KE-GrO, KE-MK and KE-ZnO increase up to 475, 430 and 364 MPa, which represent increases of 128%, 106% and 75%, respectively. The crosslinking density (ρ) can be calculated after the transition region by the following equation⁶⁷

ρ = \frac{E_{r}^{'}}{3 {RT}_{r}}

(2)

where R is the ideal gas constant

(8.3145 \frac{J}{mol \cdot K})

and

E_{r}^{'}

and T_r are the storage modulus and the temperature in the rubbery plateau region, respectively.

According to the ρ values calculated and presented in Table 2, it is possible to appreciate that the rigidity of the rubbery plateau region increases significantly with increasing the physical crosslinking, which can be attributed to the presence of the high surface area of ZnO nanoparticles, as well as the high aspect ratio clay and graphene platelets, which act as effective physical crosslinkers that restrain the localized molecular motions. The lowest value of crosslinking density was obtained for KE-CuO.

Table 2.

Viscoelastic parameters of KE and KE nanocomposites

Composition	Storage modulus in the glassy region (MPa)	Storage modulus in the rubbery plateau (MPa)	crosslinking density (ρċmol/cm³)
KE	2356.9	238.6	2.57 E-02
KE-GrO	3915.2	479.3	5.16 E-02
KE-MK	3070.4	401.7	4.32 E-02
KE-ZnO	2730.8	364.1	3.91 E-02
KE-CuO	2280.3	203.6	2.19 E-02

From Figure 4(b), it is evident that the addition of CuO and ZnO nanoparticles does not alter significantly the position of the tan δ peaks located at approximately 71℃. The peak position for the unfilled KE was located at 72℃. The previous behavior supports the idea of poor interfacial interactions between these kinds of nanoparticles and the KE bio-composite, in agreement with other studies from the literature.^47,68–70

By contrast, T_g was shifted toward a higher temperature, showing maximum peaks at 83℃ and 85℃ for KE-GrO and KE-MK, respectively. The positive shift in the tan δ peak position can be attributed to the physical interaction between the polymer and reinforcements that restrict the segmental chain motion in the KE bio-composites.^68–70 Further, KE-GrO and KE-MK displayed narrower peaks with higher amplitude compared to the rest of the materials in this study. A larger area under the tan δ curve implies the beginning of chain motions from a wide range of temperatures.

Mechanical properties

Representative stress versus strain curves corresponding to the unfilled KE and KE nanocomposites are shown in Figure 5.

Figure 5.

Tensile stress versus strain curves of unfilled kenaf/epoxidized vegetable oil (KE) and KE nanocomposites.

In the first instant, linear elastic-like behavior is illustrated in Figure 5. However, close observation allows one to identify a slight change of the slope during the unidirectional tensile tests. The first linear region represents the elastic behavior. The non-linear region, shown in the inset graph in Figure 5, is attributed to the sticking–slipping mechanism of woven composites.^71,72 In this second region, progressive elastoplastic behavior was exhibited before the sudden failure. The KE-CuO composite was the only material that revealed completely linear elastic behavior. Mechanical parameters, such as the Young’s modulus and ultimate tensile strength (UTS), obtained from the stress versus strain curves, are presented in Figure 6.

Figure 6.

Uniaxial tensile properties for unfilled kenaf/epoxidized vegetable oil (KE) and KE nanocomposites.

Regarding the Young’s modulus, the reduction of stiffness when ZnO and CuO nanoparticles are added to the KE bio-composites is evident. In contrast, stiffness increases greatly in the KE reinforced with GrO and MK nanoparticles. For the case of ZnO, several works^22,73–75 have detailed the natural tendency of ZnO nanoparticles to agglomerate, affecting the interaction with polymer chains that leads to reduced stiffness. The non-rigid CuO particles enable the non-tough-like behavior of the KE bio-composites, and the spherical-like shape nanoparticles can cause lower stiffness because of the so-called ball bearing effect, which is also related to the low crosslinking effect (density) calculated in this work during the DMA analysis.

The mechanical parameters of the KE bio-composite filled with solid ZnO and CuO particles followed a similar tendency to that observed in the DMA tests. Several authors^25,26,76 have explained the mechanisms of stiffening of FRP systems when fillers at the nano-scale level are incorporated. They underline that the mechanisms of stiffening could be attributed to the morphology of the particles, the functionality of the particle surface, the chemical nature or different interaction phenomena, which increase the crosslinking network. Kong et al.⁷⁷ studied the effect of CuO nanostructure morphology on the mechanical properties of CuO/woven carbon fiber/vinyl ester composites, finding that the higher specific surface area of nanoparticles enhanced the load transfer and load-bearing capacity. However, in the present work, the CuO morphology and surface composition were not controlled, which could avoid continuous crosslinking affecting the mechanical properties by the plasticizing effect.

On the other hand, GrO and MK have different morphologies compared with ZnO and CuO particles. They are layered structures with the ability to be exfoliated or slipped. The dispersed platelets, with their large aspect ratios and low percolation thresholds, improve the stiffness of KE bio-composites because the constrained regions formed by intercalated structures induce some restrictions to polymer chains. Furthermore, platelets provide a major contact area compared with rock-like shape nanoparticles.^45,78 It is well accepted that the interfacial microstructure plays a significant role in the stiffening of composites.^35,36,79 In this sense, surface platelets of GrO and MK could favor shrinking polymer chains because the flat thickness of this layered configuration can wrinkle and attach tightly to the polymer chains. The compaction of polymer chains leads to the toughening composites, enhancing the strength and reducing the strain before failure, as observed in Figure 5.

Regarding strength, it is well-stablished that the tensile test conducts fiber-dominated properties that depend on the great adhesion between fabrics and polymer chains. However, the addition of small amounts of nanofillers into FRP systems can improve not only the physical or chemical properties, but also mechanical and fracture behavior, being oriented to the structural applications.^80,81 The strength of the KE bio-composites is increased greatly by adding different kind of nanoparticles, as observed in Figure 6. The tensile strength of the KE bio-composite (40.56 MPa) shows a linear trend to increase by adding CuO and ZnO nanoparticles with a rock-like shape, providing an increment of strength of 13% and 27%, respectively. Meanwhile, the addition of layered particles, such as MK and GrO, improves the KE bio-composite strength by around 59% and 75%, respectively. Despite the ZnO and CuO nanoparticles having a predominant “spherical” morphology, which favors crack deflection caused by the stress concentrations in their periphery, the strength enhancement could be associated with the effective modification of nanoparticles that allows the efficient stress transfer between the functionalized fillers and the EVO matrix.⁸²

Following the ASTM D3039 – D3039M standard, a failure examination was carried out and was classified as follows: the KE biolaminate fails by fracture type AIT (angled, inside grip/tab, top); while KE-GrO, KE-MK and KE-ZnO fail by fracture type AGM (angled, gage, middle) and KE-CuO by fracture type AGB (angled, gage, bottom). For all cases, angular failure was observed and attributed to a misaligned weave, promoting a non-homogeneous stress in the transversal section. The weft and warp did not show symmetry during the tensile tests, which can be attributed to the handcraft woven fabrics, and could cause a variety of unexpected failures. However, evidence of failure for agglomerate particles or bubbling was not observed.

Voids or bubbles can be formed due to incomplete resin filling, air entrapment or disrupted resin flow caused by the presence of nanoparticles in the epoxy resin. Furthermore, voids can adopt different shapes, sizes and orientations that depend not only on the intrinsic nature of the system (fiber/resin/nanoparticle), but also on the manufacturing process parameters, such as pressure or temperature. X-ray CT is becoming increasingly important among the non-destructive inspection techniques for FRP composites because it uses the material densities to detect, and count, the presence of bubbles, particles or defects in the composite laminate, and is a useful tool for determining the volume fraction.^83,84 Figure 7 shows the volume fractions of the unfilled KE and KE nanocomposites determined by tomography.

Figure 7.

Representative histograms of the volume fraction of each bio-laminate obtained from computed tomography analysis.

The porosity level present in KE-GrO was considerably higher than the rest of the KE nanocomposites evaluated in this work, and could be attributed to agglomerated particles and non-effective dispersion of GrO sheets, which alter the fraction volume and impact on the mechanical properties. In contrast, KE-MK showed a lower tendency to produce voids, which can be related to exfoliated and well-dispersed platelets that promoted excellent mechanical properties.

Conclusions

Kenaf bio-composites filled with different kind of nanoparticles were successfully prepared in this work using a hybrid manufacturing process. The nanoparticles were prepared and synthesized according to their nature. FTIR spectra confirmed the presence of functional groups that should allow the enhancement of the interface interaction between the fillers and polymer chains. The structural characterization performed by XRD revealed that GrO present a lack of interconnection of the carbon structure with oxygen atoms, which resulted in agglomerated particles detected by tomography. The thermal treatment of calcined kaolin produced a MK structure, and the synthesis of metallic particles resulted in a wurtzite-like structure typical of ZnO and a monoclinic phase of tenorite structure for CuO.

The laminar structure of GrO and MK promoted crystal sizes lower than metallic oxides, as determined by the Debye–Scherrer equation. The morphologies of the particles seem to influence the viscoelastic and mechanical properties of KE bio-composites. The high aspect ratio and dispersion of GrO and MK platelets restrained the localized molecular motions and promoted the higher values of the storage modulus. The presence of CuO and ZnO nanoparticles did not alter the position of the tan δ peaks because of low interfacial interactions between the metallic oxides and the KE bio-composite.

The higher specific surface area of GrO and MK platelets enhanced the stiffness of KE bio-composites. Nonetheless, the load transfer capacity of the fillers evaluated in this work was effective, improving greatly the biolaminate strength.

CT observations allowed one to determine that the porosity level of KE-GrO was considerably higher than the rest of the KE nanocomposites, and was attributed to agglomerated particles and non-effective dispersion of GrO sheets, which altered the fraction volume and impacted the mechanical properties. In contrast, KE-MK showed a lower tendency to produce voids because of exfoliated and well-dispersed platelets that promoted good mechanical performance.

Footnotes

Authors' contributions

AV Rentería-Rodríguez: data analysis, writing, graphs, discussion.

EA Franco-Urquiza: data analysis, writing, graphs, discussion, funding.

Acknowledgements

FTIR and DMA analysis were performed by Eng. Raul Samir Saleme and Rodrigo Ramirez.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported by the Secretary of Public Education (SEP) and National Council of Science and Technology (CONACYT) through the Basic Science Fund (SEP-CONACYT Ciencia Básica) (Grant No. 257458).

AV Rentería-Rodríguez thanks CONACYT for the scholarship to pursuit her master’s degree.

Edgar A Franco-Urquiza conveys special appreciation to the “CONACYT Researchers Program (Cátedras CONACYT)”.

ORCID iD

EA Franco-Urquiza

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